基于土壤含水率的骤发干旱和缓慢干旱时空特征分析

骤发干旱是近年来全球不同地区频繁发生的极端事件,与缓慢干旱相比,其爆发异常迅速,严重威胁粮食农业安全和生态环境健康。为探究骤发与缓慢干旱的时空特征,该研究基于再分析产品土壤含水率数据,提取中国1979-2018年干旱事件及旱情开始阶段历时,分析骤发干旱与缓慢干旱在发生频次、影响范围上的差异,并以典型干旱事件为例,探讨二者的联系。结果表明:1)总体上,中国干旱事件呈长历时、大范围的发展趋势;2)旱情开始阶段的分析表明,中国南方地区开始历时总体比北方偏短,典型骤旱事件主要集中在内蒙古中东部、西南地区、珠江上游以及长江下游;缓慢干旱则主要集中在西北、新疆地区,并且其覆盖范围在2005年以后增幅明显;3)近10年2场典型干旱事件的时空演变轨迹表明,骤发干旱与缓慢干旱在旱情演进过程中无明显界限,二者既可以独立存在,也可能相伴发生。该研究可为认识干旱形成机理以及旱情监测技术的发展提供参考。

Analysis of spatio-temporal characteristics of flash drought and slowly-evolving drought using soil moisture percentile

Abstract: Flash drought is one kind of extreme hydro-meteorological events that frequently occur worldwide in recent years. Different from traditional slowly-evolving droughts, flash droughts unusually have rapid onset, threatening food and agricultural security, and the health of eco-environment. To compare the spatio-temporal characteristics of flash drought and slowly evolving drought, the soil moisture data were extracted from the ERA-Interim reanalysis product (at a depth of 0-100 cm) with a spatial resolution of 0.25°and a temporal coverage from 1979 to 2018. Records of the soil moisture data were converted into weekly values. Seven candidate probability distribution functions (including Gamma, Beta, logarithmic, logical logarithmic, Weibull, extreme value, and generalized extreme value distributions) were employed to fit the weekly data series. The optimal probability distribution function for calculating soil moisture percentile was selected according to the Kolmogorov-Smirnov test (KS test) and two evaluation coefficients, i.e., the root mean square error and bias. For drought event identification, the 40% and 20% of soil moisture percentile were used as the upper limit and the lower limit, respectively, to ensure the identified drought events really falling into drought associated with vegetation stress. The onset duration was determined as the period when soil moisture percentile declines persistently and maintains a rather low value. The differences between flash droughts and slowly-evolving droughts in terms of drought frequency and drought area were analyzed. The relationships between flash droughts and slowly-evolving droughts were investigated based on the spatio-temporal patterns of two typical drought events. The results showed that droughts with long duration (more than 26 weeks) were more frequent in the north of the Yangtze River. In contrast, the south of the Yangtze River suffered few droughts and short-term drought. Different from drought events with short duration, the time series of affected area percentage for drought events with long duration (26 weeks or longer) presented significant increments from 2005 to 2015, especially in 2010 when more than 40% of the area suffered drought, suggesting a general trend of droughts with long duration and enlarged area extent. During the drought onset stage, the duration of drought of the south region was shorter than that of the north region. Typical flash drought events occurred in the middle and eastern part of Inner Mongolia, southwest China, the upper reaches of the Pearl River, and lower reaches of the Yangtze River, while slowly-evolving droughts happened mainly in the northwest China, and some areas in Xinjiang. The area percentage of flash drought was comparable to that of the slowly-evolving drought before 2000, after that, the affected area of slowly-evolving drought expanded significantly up to twice that of flash drought. The spatio-temporal migration paths of the 2010 autumn drought and the 2017 summer drought showed that the flash drought and slowly-evolving drought may develop independently, and may also present a co-existing relationship. Finally, the in-situ soil moisture measurements from the International Soil Moisture Network (ISMN) were employed for validation of the results. The spatial pattern of moisture was similar with that of the ERA-Interim reanalysis product. For example, soil moisture in humid and semi-humid zones decreased much quickly than in arid and semi-arid zones during a drought episode. This may be related to the climate condition, soils, and vegetation characteristics of the underlying surface. The soil moisture data of the ERA-Interim reanalysis product is capable of capturing the moisture dynamics during the onset stage of drought, and could be an alternative for flash drought monitoring and assessments. The results can provide some important information for understanding the mechanisms of drought and the development of drought monitoring techniques.